Ukrainian troops listen to President Petro Poroshenko from atop an armored personnel carrier at their base in Devhenke, Ukraine, on July 8. / Evgeniy Maloletka, AP

by Oren Dorell, USA TODAY

by Oren Dorell, USA TODAY

bā?¢ Corrections & Clarifications: A previous version of this article misidentified Damon Wilson's affiliation. He is executive vice president of the Atlantic Council.

Ukrainian security forces encircled and shelled separatist strongholds Tuesday in the eastern cities of Donetsk and Luhansk, continuing a swift offensive to retake the center of the pro-Russian insurgency.

The Ukrainian military has overcome deficiencies in equipment, training, strategy and morale, and Russian President Vladimir Putin has refrained from launching a full-scale invasion across the Ukrainian border to rescue the rebellion he helped kindle.

Russian Foreign Minister Sergei Lavrov called Tuesday for an immediate cease-fire. "We need to throw aside everything to immediately stop the use of force," Lavrov said.

Ukrainian Defense Minister Valery Heletey said earlier that there will be no cease-fire until rebels put down their weapons. Ukrainian forces seized Russian-made anti-tank and anti-aircraft missiles left behind by fleeing separatists in the city of Slovyansk, where they were routed by government forces Saturday, Heletey said on the ministry's website.

Ukrainian President Petro Poroshenko spoke in the central square Tuesday in Slovyansk, which was home to the rebels' main headquarters in eastern Ukraine's industrialized Donbass region. The city that was once "a symbol of terror and violence" is now a symbol of "a free Donbass, and I thank you for that," Poroshenko said, according to Reuters.

Ukrainian forces' success depends on Russia staying out of the fight. The European Union has threatened more sanctions in response to incursions by Moscow.

Though Russia continues to send fighters and small arms across the border, the level of intervention has declined in the past few weeks, in part because "the Ukrainians called Russia's bluff," said Janusz Bugajski, a senior fellow at the Center for European Policy Analysis in Washington. "The Ukrainians have been more effective, and the Russians haven't upped the ante and haven't increased their supplies" to the separatists.

Putin is "in a difficult situation," Bugajski said. "What does he do next if the rebels are ousted completely? It's a big decision I think for him in the next few days."

Ukrainian forces sought to control a 6-mile zone along the border and will shift to a different set of tactics for operations aimed at retaking the rebel nerve centers of Luhansk and Donetsk, said Damon Wilson, an analyst at the Atlantic Councilwho returned recently from meetings with security leaders in Ukraine.

"You can't go in with heavy metal into the population centers," said Wilson, a Russia and Ukraine expert in the administrations of Presidents George W. Bush and Bill Clinton. "I think you'll see a change in tactics to minimize as much as possible damage and civilian casualties."

Ukraine's counterstrike, which came after Poroshenko ended a 10-day cease-fire June 30, started with a bombing campaign and a pincer movement from the north and south to retake border crossings, says Phillip Karber, a former U.S. Defense Department official under President Ronald Reagan who's been meeting with Ukrainian military officials in Kiev and traveling along the front in eastern Ukraine as an observer.

The move is risky because it leaves Ukrainian forces exposed to a Russian attack from the east should Putin decide to make a move, but it shows that Ukrainian commanders and ground fighters have quickly adapted a scantly supplied air force and military into a keen fighting force, Karber says.

Ukraine used aerial surveillance gathered during the cease-fire and enough ground-attack aircraft cobbled together from cannibalized parts to launch strafing and bombing runs against separatist encampments the night the cease-fire ended.

"Because previous attacks had usually been in support of ground operations and done piecemeal, the proxy forces didn't seem to see this coming, and their open stored concentrations and fixed targets suffered because of it," Karber said in an e-mail from Ukraine.

"If the Russians decide to launch an air offensive," the Ukrainian air force "is in deep trouble," Karber said. "They have little early warning, too few operating bases, no shelters, aircraft not dispersed and no allocated rapid runway repair assets."

On the ground, Ukraine's first goal was to seal the 350-mile border between the Luhansk and Donetsk regions and Russia. That has been only partially effective. A company of Russian-supplied tanks crossed the border Friday by traveling over plowed fields, Karber said, citing Ukrainian military sources.

The next objective, according to front-line Ukrainian commanders, is to take Donetsk and Luhansk, the separatists' political and military centers, home to their organizing, recruitment and logistics operations, Karber said.

Ukraine will seek to "permanently occupy them with sufficient force to control the surrounding areas," Karber said. "This is not as easy as it sounds, and getting into those cities is only half the battle." Ukrainian forces will be at risk of being cut off from their own supplies.

Despite evidence of Russian support for the separatists and Russian special forces veterans conducting operations, 90% of the separatists are locals who enjoy local support, Karber estimates.

If Russian battalions adjacent to the Donetsk and Luhansk regions "suddenly crossed the border under guise of 'peacekeeping,' that would catch the current Ukrainian offensive from the flank and rear," Karber said. Poroshenko's strategy for pacifying the east "would be in shambles."